The present invention generally relates to inorganic solid lubricant coatings, and to methods for forming such solid lubricant coatings.
Compliant foil air bearings (hereafter, foil bearings) are at the forefront of oil-free turbomachinery, in which gas turbine engines with hydrodynamic bearings use air instead of oil as the working fluid. Foil bearings have several commercial applications, such as air cycle machines, turbocompressors, and microturbines, as well as aircraft gas turbine engines.
The rotating shaft of a foil bearing may act as an air pump to create a thin layer of high pressure air between the shaft or journal and the top foil. As a result, during normal operation air serves as lubricant for the bearing. To protect the bearing during start-up and shut down, i.e., in the absence of high pressure air between the shaft and top foil, solid lubricants have been used. Such solid lubricants have typically been applied to the top foil, but may also be applied to the shaft.
Prior art solid lubricants for foil bearings have been applied to a substrate (e.g., top foil or shaft) as liquids, for example, by spraying the liquid on the substrate surface, or by dipping the substrate in the liquid. Such liquids of prior art solid lubricants have used organic polymer constituents, as a lubricant per se, or as a binder. For example, U.S. Pat. No. 4,435,839 (to Gu, et al.) discloses a liquid coating comprising an organic thinning liquid, an organic polyimide binder, and an organic wetting agent; as well as a process for applying the liquid to a substrate by spraying nine (9) sub-coats of the liquid, with a series of intermittent curing steps (one hour in an oven), and intermittent sanding steps.
Foil bearings using conventional organic-based solid lubricants are typically limited to an operating temperature of not more than about 350° C., and therefore cooling of the foil bearing components during operation is required. Such cooling consumes fuel and increases both operating cost and weight.
An inorganic solid lubricant was described in U.S. Pat. No. 5,866,518 (to Dellacorte et al.), which discloses a self-lubricating composite comprising a chromium oxide (e.g., chromic oxide, Cr2O3), a metal binder comprising a Cr/Ni or Cr/Co alloy, a metal fluoride, and, optionally, a metal lubricant. A milled powder mix of the composite is deposited on the substrate by plasma spraying, and the like; or, free-standing bodies of the composite may be formed by cold isostatic pressing or hot isostatic pressing.
As can be seen, there is a need for an inorganic solid lubricant adapted for operating temperatures in excess of those tolerated by organic-based solid lubricants. There is a further need for an inorganic solid lubricant which can be readily and inexpensively applied to a substrate in a simple and straightforward process, such that processing costs are decreased, and productivity is increased. There is also a need for a process for coating superalloy components or other substrates with an inorganic solid lubricant by applying a precursor material to the substrate, wherein the precursor material includes at least one inorganic bonding component, and wherein the inorganic bonding components are melted on the substrate surface, at a temperature insufficient to thermally damage the substrate, whereby the solid lubricant coating is bonded to the substrate surface.